Innovative and simplified oil well service tools that offer high reliability must be developed due to the extreme economy by which oil field services must now operate. Whenever pipe is to be cut, recovered, or freed, experience has shown that the cut produced by a chemical cutter offers the least trouble, smallest overall expense and the highest success in the recovery operation. This is because the cut is not flared, has no burrs or sharp projections, and the inside and outside diameters around the cut are not changed. Therefore, an overshot can be deployed downhole and be easily placed over the outside of the pipe string without incurring the additional cost of a milling operation in order to recover the pipe.
Chemical cutters can be used to great advantage in the application of chemicals to cut, sever or perforate downhole pipe. For example, in U.S. Pat. No. 2,918,125 to Sweetman halogen fluorides are employed in jet streams impinging on the pipe to sever or perforate the pipe. The attendant reaction is highly exothermic and the pipe is rapidly penetrated. Additionally, as disclosed in U.S. Pat. No. 4,619,318 to Terrell and Pratt, objects may be perforated or in some instances completely dissolved downhole by a chemical cutter, with no debris left in the well. The halogen fluoride used in the chemical cutter produces a chemical reaction that completely dissolves the pipe in the cut area. Since there are no expendable mechanical parts of the chemical cutter, no debris is left downhole.
During the course of the cutting operation, the cutting tool must be anchored at the desired location within the well. This is particularly the case where the cutting tool is run into the well on a wireline. One technique for anchoring the tool employs the use of fluid pressure from a suitable source to both activate the anchoring means and dispel cutting fluid from the tool against the surface to be severed or otherwise cut. For example, an anchoring means is disclosed in U.S. Pat. No. 3,076,507 to Sweetman wherein the chemical cutter anchoring means comprises "button slips" that are radially projected by the chemical cutter tool's pressurizing medium to anchor the tool to the wellbore casing. However, this anchoring means fails to positively centralize the tool and the "button slips" can occasionally penetrate through old pipe, thus sticking the tool downhole. In addition, should the tool be accidentally discharged above the ground, the "button slips" could be discharged at high velocities similar to bullets from a gun, which could result in the injury of operational personnel or equipment.
U.S. Pat. No. 4,125,161 to Chammas discloses another anchoring means for a chemical cutter in which gas from a propellant charge displaces a piston to cam one or more wedges outwardly against the tubing string or object to be severed. The gas from the propellant charge is also employed to force the cutting chemical into contact with a preignitor and thence outwardly through ports into contact with the pipe to be severed. The wedges of this invention afford inadequate anchoring and centralization occasionally when severing pipe downhole. The wedges of this anchoring system offer limited range and limited anchoring capabilities, permitting the tool to be occasionally shot up hole while a cut is being made.
A particularly effective chemical cutting tool is disclosed in U.S. Pat. No. 4,345,646 to Jamie B. Terrell. In this tool a chemical module assembly is located intermediate to a propellant assembly and a cutting head assembly. Gas pressure generated by the ignition of a propellant charge is employed to rapidly move a slip array against a slip expander, during which time the cutting action takes place. The slip array is then rapidly retracted by means of a biasing mechanism. The slip segments are disposed in the array in a manner to provide maximum utilization of surface area of the slip assembly for engaging the surrounding pipe. A tension spring, referred to in this patent as a "garter spring", is provided around the slip segments in order to bias the slip segments inwardly against the slip shaft. The garter spring is disposed within slotted recesses within the slip segments located between the teeth of the slip segments and the heads of the slip segments.
U.S. Pat. No. 4,415,029 to Terrell discloses a chemical cutting tool having another form of slip actuating means and slip array configuration. The well tool anchoring means of this patent comprises a slip array located on a slip shaft and interposed between suitable actuation means and slip expansion means. The slip segments are biased inwardly by means of cantilever springs secured to a structural member of the cutting tool and projecting into engagement with downward movement of the slip segments. Preferably the slip segments are arranged in the array in a diametrically asymmetrical relationship. Also disclosed in aforementioned U.S. Pat. No. 4,415,029 to Terrell is a large external spring biasing means to force the slip piston into the retracted position.
Another cutting tool is disclosed in U.S. Pat. No. 4,620,591 to Terrell and Pratt. In this patent, the inwardly slip biasing function is provided by a mechanism which includes expandable slip segments each having a raised ridge in each side of the segments. Each ridge is mounted through annular grooves which in cooperation with ball bearings serve to splay the slips outwardly in response to a downward force. This patent also discloses the use of an external spring which is threaded onto the slip shaft and a top slip subassembly in which the slip segments of the slip array are mounted.